[ceph.git] / ceph / src / boost / libs / preprocessor / doc / topics / techniques.html

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<h4>Techniques</h4>
<div>
	The preprocessor metaprogramming techniques are presented in example format.
</div>

<h4>Example<u> - Use a local macro to avoid small scale repetition.</u></h4>
<div class="code"><pre>
#define BOOST_PP_DEF(op) /* ..................................... */ \
   template&lt;class T, int n&gt;                                          \
   vec&lt;T, n&gt; operator op ## =(vec&lt;T, n&gt; lhs, const vec&lt;T, n&gt;& rhs) { \
      for (int i = 0; i &lt; n; ++i) {                                  \
         lhs(i) op ## = rhs(i);                                      \
      }                                                              \
   }                                                                 \
   /**/

BOOST_PP_DEF(+)
BOOST_PP_DEF(-)
BOOST_PP_DEF(*)
BOOST_PP_DEF(/)

#undef BOOST_PP_DEF
</pre></div>
<div>
	<b>Tip:</b>&nbsp; It is usually okay to use a standard macro name like <code>BOOST_PP_DEF</code> for this kind of code
	because the macro is both defined and undefined in the immediate site of its use.
</div>
<div>
	<b>Tip:</b>&nbsp; It is easier to verify proper use of the line continuation operator when they are aligned.
</div>
<div>
	<b>Notes:</b>&nbsp; You can extend this example by defining more and different kinds of operators.&nbsp;
	Before doing so, consider using the <i>algebraic categories</i> technique introduced in <a href="../bibliography.html#barton">[Barton]</a>
	or a <i>layered architecture</i> (see for instance <a href="../bibliography.html#czarnecki">[Czarnecki]</a>).&nbsp;
	However, at some point you must type the operator tokens <code>*</code>, <code>/</code>, <code>+</code>, <code>-</code>, etc.,
	because it is impossible to generate them using templates.&nbsp;
	The resulting <i>categorical repetition</i> of tokens can be eliminated by using preprocessor metaprogramming.
</div>

<h4>Example<u> - Use BOOST_PP_EMPTY as an unused parameter in local macro instantiations.</u></h4>
<div class="code"><pre>
#define BOOST_PP_DEF(cv) /* ... */ \
   template&lt;class base&gt;            \
   cv() typename implement_subscript_using_begin_subscript&lt;base&gt;::value_type& \
   implement_subscript_using_begin_subscript&lt;base&gt;::operator[](index_type i) cv() { \
      return base::begin()[i];     \
   }                               \
   /**/

   BOOST_PP_DEF(BOOST_PP_EMPTY)
   BOOST_PP_DEF(BOOST_PP_IDENTITY(const))
</pre></div>
<div>
	<b>How:</b>&nbsp; BOOST_PP_EMPTY() expands to nothing and can be used as an unused parameter.
</div>
<div>
	<b>Note:</b>&nbsp; BOOST_PP_EMPTY with the () never gets expanded.&nbsp;
	The () is necessary to invoke a function-like macro.
</div>
<div>
	<b>Caveat:</b>&nbsp; You cannot safely use concatenation while using BOOST_PP_EMPTY().
</div>
<div>
	<b>Tip:</b>&nbsp; Occasionally, one or two lines are considerably longer than the rest.&nbsp;
	It can often save some work to <i>not</i> align all the line continuation operators without making the code too unreadable.
</div>
<div>
	<b>Tip:</b>&nbsp; Use syntax highlighting on preprocessor metaprogramming macro identifiers such as:
	<ul>
		<li>BOOST_PP_DEF</li>
		<li>BOOST_PP_EMPTY</li>
		<li>BOOST_PP_REPEAT</li>
		<li>...</li>
	</ul>
	It can greatly improve readability.
</div>

<h4>Example<u> - Use BOOST_PP_CAT instead of ## when necessary.</u></h4>
<div class="code"><pre>
#define STATIC_ASSERT(expr) \
   enum { BOOST_PP_CAT(static_check_, __LINE__) = (expr) ? 1 : -1 }; \
   typedef char \
      BOOST_PP_CAT(static_assert_, __LINE__)[BOOST_PP_CAT(static_check_, __LINE__)] \
   /**/

// ...

STATIC_ASSERT(sizeof(int) &lt;= sizeof(long));
</pre></div>
<div>
	<b>Why:</b>&nbsp; Macro expansion proceeds recursively in "layers."&nbsp;
	Token pasting prevents the preprocessor from performing macro expansion,
	therefore it is often necessary to delay token concatenation.
</div>

<h4>Example<u> - Use BOOST_PP_STRINGIZE instead of # whenever necessary.</u></h4>
<div class="code"><pre>
#define NOTE(str) \
   message(__FILE__ "(" BOOST_PP_STRINGIZE(__LINE__) ") : " str) \
   /**/

// ...

#pragma NOTE("TBD!")
</pre></div>
<div>
	<b>Why:</b>&nbsp; Macro expansion proceeds recursively in "layers."&nbsp;
	Stringization prevents the preprocessor from performing macro expansion, therefore it is often necessary to delay stringization.
</div>

<h4>Example<u> - Use BOOST_PP_ENUM_PARAMS (and its variants) or BOOST_PP_REPEAT and BOOST_PP_COMMA_IF to avoid <i>O</i>(<i>n</i>) repetition on lists in general.</u></h4>
<div class="code"><pre>
struct make_type_list_end;

template&lt;
   BOOST_PP_ENUM_PARAMS_WITH_A_DEFAULT(
      MAKE_TYPE_LIST_MAX_LENGTH, 
      class T, 
      make_type_list_end
   )
&gt;
struct make_type_list {
   private:
      enum { end = is_same&lt;T0, make_type_list_end&gt;::value };
   public:
      typedef typename type_if&lt;
         end, type_cons_empty,
         type_cons&lt;
            T0,
            typename type_inner_if&lt;
               end, type_identity&lt;end&gt;,
               make_type_list&lt;
                  BOOST_PP_ENUM_SHIFTED_PARAMS(
                     MAKE_TYPE_LIST_MAX_LENGTH,
                     T
                  )
               &gt;
            &gt;::type
         &gt;
      &gt;::type type;
};
</pre></div>
<div>
	<b>How:</b>&nbsp; BOOST_PP_REPEAT uses simulated recursion (pseudo code):
</div>
<div><pre>
#define BOOST_PP_REPEAT(n, m, p) BOOST_PP_REPEAT ## n(m, p)

#define BOOST_PP_REPEAT0(m, p)
#define BOOST_PP_REPEAT1(m, p) m(0, p)
#define BOOST_PP_REPEAT2(m, p) m(0, p) m(1, p)
#define BOOST_PP_REPEAT3(m, p) BOOST_PP_REPEAT2(m, p) m(2, p)
#define BOOST_PP_REPEAT4(m, p) BOOST_PP_REPEAT3(m, p) m(3, p)
// ...
</pre></div>
<div>
	<i>Note:&nbsp; This is no longer how BOOST_PP_REPEAT is implemented, so the code above is illustrative only!&nbsp;</i>
</div>
<div>
	BOOST_PP_ENUM_PARAMS and its variations use BOOST_PP_REPEAT.&nbsp;
	BOOST_PP_COMMA_IF(I) expands to a comma if I != 0.&nbsp;
	BOOST_PP_INC(I) essentially expands to "I+1," and BOOST_PP_DEC(I) essentially expands to "I-1.".
</div>

<h4>Example<u> - Use a <i>conditional macro definition</i> to enable user configuration of code repetition based on need rather than some "reasonable" upper limit.</u></h4>
<div class="code"><pre>
#ifndef MAKE_TYPE_LIST_MAX_LENGTH
#define MAKE_TYPE_LIST_MAX_LENGTH 8
#endif
</pre></div>
<div>
	Now the user can configure the <code>make_type_list</code> primitive without modifying library code.
</div>

<h4>Example<u> - Use BOOST_PP_REPEAT and a <i>token look-up function</i> to eliminate categorical repetition.</u></h4>
<div class="code"><pre>
// CAVEAT:  My compiler is not standard on arithmetic types.
#define ARITHMETIC_TYPE(I) ARITHMETIC_TYPE ## I

#define ARITHMETIC_TYPE0    bool
#define ARITHMETIC_TYPE1    char
#define ARITHMETIC_TYPE2    signed char
#define ARITHMETIC_TYPE3    unsigned char
#define ARITHMETIC_TYPE4    short
#define ARITHMETIC_TYPE5    unsigned short
#define ARITHMETIC_TYPE6    int
#define ARITHMETIC_TYPE7    unsigned int
#define ARITHMETIC_TYPE8    long
#define ARITHMETIC_TYPE9    unsigned long
#define ARITHMETIC_TYPE10   float
#define ARITHMETIC_TYPE11   double
#define ARITHMETIC_TYPE12   long double

#define ARITHMETIC_TYPE_CNT 13

// ...

#define BOOST_PP_DEF(z, I, _) /* ... */ \
   catch (ARITHMETIC_TYPE(I) t) {       \
      report_typeid(t);                 \
      report_value(t);                  \
   }                                    \
   /**/

BOOST_PP_REPEAT(ARITHMETIC_TYPE_CNT, BOOST_PP_DEF, _)

#undef BOOST_PP_DEF
</pre></div>
<div>
	<b>Note:</b>&nbsp; The repetition of the above example can be eliminated using template metaprogramming <a href="../bibliography.html#czarnecki">[Czarnecki]</a> as well.&nbsp;
	However categorical repetition of operator tokens cannot be completely eliminated by using template metaprogramming.
</div>

<h4>Example<u> - Use BOOST_PP_REPEAT to avoid <i>O</i>(<i>n</i>*<i>n</i>) repetition.</u></h4>
<div class="code"><pre>
#ifndef MAX_VEC_ARG_CNT
#define MAX_VEC_ARG_CNT 8
#endif

// ...

#define ARG_FUN(z, i, _) BOOST_PP_COMMA_IF(i) T a ## i
#define ASSIGN_FUN(z, i, ) (*this)[i] = a ## i;

#define DEF_VEC_CTOR_FUN(z, i, _) /* ... */ \
   vec(BOOST_PP_REPEAT(i, ARG_FUN, _)) { \
      BOOST_PP_REPEAT(i, ASSIGN_FUN, _)  \
   }                                     \
   /**/

BOOST_PP_REPEAT(BOOST_PP_INC(MAX_VEC_ARG_CNT), DEF_VEC_CTOR_FUN, _)

#undef ARG_FUN
#undef ASSIGN_FUN
#undef DEF_VEC_CTOR_FUN

// ...
</pre></div>
<div>
	<b>How:</b>&nbsp; BOOST_PP_REPEAT is implemented is a special way to enable <i>automatic recursion</i>.
</div>

<h4>Example<u> - Use BOOST_PP_IF to implement special case for the first element.</u></h4>
<div class="code"><pre>
#define COMMA_IF(c) \
   BOOST_PP_IF(c, BOOST_PP_COMMA, BOOST_PP_EMPTY)() \
   /**/

BOOST_PP_IF(0, true, false) == false;
BOOST_PP_IF(1, true, false) == true;
</pre></div>
<div>
	BOOST_PP_IF enables convenient generation of lists using BOOST_PP_REPEAT.
</div>
<div>
	<b>Note:</b>&nbsp; <i>THEN</i> and <i>ELSE</i> don't have to be macros.&nbsp;
	However, if at least one of them is a function-like macro, and you want it be expanded conditionally,
	you have to make the other parameter a function-like macro too.&nbsp;
	This can often be done using BOOST_PP_IDENTITY.&nbsp;
	Consider the following example (by Aleksey Gurtovoy):
</div>
<div><pre>
#define NUMBERED_EXPRESSION(i, x) /* ... */ \
   BOOST_PP_IF(                             \
      i,                                    \
      BOOST_PP_IDENTITY(x ## i)             \
      BOOST_PP_EMPTY                        \
   )()                                      \
   /**/
</pre></div>
<div>
	<b>Note:</b>&nbsp; Like in the above implementation of COMMA_IF, the result of BOOST_PP_IF is often invoked and not the <i>THEN</i> and <i>ELSE</i> parameters.&nbsp;
	If the parameters were invoked, the code would not expand correctly, because the BOOST_PP_EMPTY parameter would get expanded
	to nothing before the <b>BOOST_PP_IF</b> would be properly expanded.
</div>
<div>
	<b>How:</b>&nbsp; BOOST_PP_IF is defined for the entire repeat range (psuedo code):
</div>
<div><pre>
#define BOOST_PP_IF(c, THEN, ELSE) BOOST_PP_IF ## c(THEN, ELSE)

#define BOOST_PP_IF0(THEN, ELSE) ELSE
#define BOOST_PP_IF1(THEN, ELSE) THEN
#define BOOST_PP_IF1(THEN, ELSE) THEN
// ...
</pre></div>

<h4>Example:<u>&nbsp; Use arithmetic, logical, and comparison operations when necessary.</u></h4>
<div class="code"><pre>
#define SPECIAL_NUMBERED_LIST(n, i, elem, special) \
   BOOST_PP_ASSERT_MSG(                     \
      BOOST_PP_LESS(i, n),                  \
      bad params for SPECIAL_NUMBERED_LIST! \
   )                                        \
   BOOST_PP_ENUM_PARAMS(i, elem)            \
   BOOST_PP_COMMA_IF(i) special             \
   BOOST_PP_REPEAT(                         \
      BOOST_PP_SUB(BOOST_PP_DEC(n), i),     \
      SPECIAL_NUMBERED_LIST_HELPER,         \
      (elem, i)                             \
   )                                        \
   /**/

#define SPECIAL_NUMBERED_LIST_HELPER(z, i, elem_base) \
   ,                                          \
   BOOST_PP_CAT(                              \
      BOOST_PP_TUPLE_ELEM(2, 0, elem_base),   \
      BOOST_PP_ADD(                           \
         i,                                   \
         BOOST_PP_TUPLE_ELEM(2, 1, elem_base) \
      )                                       \
   )                                          \
   /**/

SPECIAL_NUMBERED_LIST(3, 0, E, S)
SPECIAL_NUMBERED_LIST(3, 1, E, S)
SPECIAL_NUMBERED_LIST(3, 2, E, S)
SPECIAL_NUMBERED_LIST(3, 3, E, S)
</pre></div>

<hr size="1">
<div style="margin-left: 0px;">
Commit	Line	Data
7c673cae FG	1	<html>
	2	<head>
	3	<title>techniques.html</title>
	4	<link rel="stylesheet" type="text/css" href="../styles.css">
	5	<style>
	6	u { font-weight: normal; text-decoration: none; }
	7	</style>
	8	</head>
	9	<body>
	10	<h4>Techniques</h4>
	11	<div>
	12	The preprocessor metaprogramming techniques are presented in example format.
	13	</div>
	14
	15	<h4>Example<u> - Use a local macro to avoid small scale repetition.</u></h4>
	16	<div class="code"><pre>
	17	#define BOOST_PP_DEF(op) /* ..................................... */ \
	18	template<class T, int n> \
	19	vec<T, n> operator op ## =(vec<T, n> lhs, const vec<T, n>& rhs) { \
	20	for (int i = 0; i < n; ++i) { \
	21	lhs(i) op ## = rhs(i); \
	22	} \
	23	} \
	24	/**/
	25
	26	BOOST_PP_DEF(+)
	27	BOOST_PP_DEF(-)
	28	BOOST_PP_DEF(*)
	29	BOOST_PP_DEF(/)
	30
	31	#undef BOOST_PP_DEF
	32	</pre></div>
	33	<div>
	34	<b>Tip:</b>  It is usually okay to use a standard macro name like <code>BOOST_PP_DEF</code> for this kind of code
	35	because the macro is both defined and undefined in the immediate site of its use.
	36	</div>
	37	<div>
	38	<b>Tip:</b>  It is easier to verify proper use of the line continuation operator when they are aligned.
	39	</div>
	40	<div>
	41	<b>Notes:</b>  You can extend this example by defining more and different kinds of operators.
	42	Before doing so, consider using the <i>algebraic categories</i> technique introduced in <a href="../bibliography.html#barton">[Barton]</a>
	43	or a <i>layered architecture</i> (see for instance <a href="../bibliography.html#czarnecki">[Czarnecki]</a>).
	44	However, at some point you must type the operator tokens <code>*</code>, <code>/</code>, <code>+</code>, <code>-</code>, etc.,
	45	because it is impossible to generate them using templates.
	46	The resulting <i>categorical repetition</i> of tokens can be eliminated by using preprocessor metaprogramming.
	47	</div>
	48
	49	<h4>Example<u> - Use BOOST_PP_EMPTY as an unused parameter in local macro instantiations.</u></h4>
	50	<div class="code"><pre>
	51	#define BOOST_PP_DEF(cv) /* ... */ \
	52	template<class base> \
	53	cv() typename implement_subscript_using_begin_subscript<base>::value_type& \
	54	implement_subscript_using_begin_subscript<base>::operator[](index_type i) cv() { \
	55	return base::begin()[i]; \
	56	} \
	57	/**/
	58
	59	BOOST_PP_DEF(BOOST_PP_EMPTY)
	60	BOOST_PP_DEF(BOOST_PP_IDENTITY(const))
	61	</pre></div>
	62	<div>
	63	<b>How:</b>  BOOST_PP_EMPTY() expands to nothing and can be used as an unused parameter.
	64	</div>
65	<div>
66	<b>Note:</b>  BOOST_PP_EMPTY with the () never gets expanded.
67	The () is necessary to invoke a function-like macro.
68	</div>
69	<div>
70	<b>Caveat:</b>  You cannot safely use concatenation while using BOOST_PP_EMPTY().
71	</div>
72	<div>
73	<b>Tip:</b>  Occasionally, one or two lines are considerably longer than the rest.
74	It can often save some work to <i>not</i> align all the line continuation operators without making the code too unreadable.
75	</div>
76	<div>
77	<b>Tip:</b>  Use syntax highlighting on preprocessor metaprogramming macro identifiers such as:
78	<ul>
79	<li>BOOST_PP_DEF</li>
80	<li>BOOST_PP_EMPTY</li>
81	<li>BOOST_PP_REPEAT</li>
82	<li>...</li>
83	</ul>
84	It can greatly improve readability.
85	</div>
86
87	<h4>Example<u> - Use BOOST_PP_CAT instead of ## when necessary.</u></h4>
88	<div class="code"><pre>
89	#define STATIC_ASSERT(expr) \
90	enum { BOOST_PP_CAT(static_check_, __LINE__) = (expr) ? 1 : -1 }; \
91	typedef char \
92	BOOST_PP_CAT(static_assert_, __LINE__)[BOOST_PP_CAT(static_check_, __LINE__)] \
93	/**/
94
95	// ...
96
97	STATIC_ASSERT(sizeof(int) <= sizeof(long));
98	</pre></div>
99	<div>
100	<b>Why:</b>  Macro expansion proceeds recursively in "layers."
101	Token pasting prevents the preprocessor from performing macro expansion,
102	therefore it is often necessary to delay token concatenation.
103	</div>
104
105	<h4>Example<u> - Use BOOST_PP_STRINGIZE instead of # whenever necessary.</u></h4>
106	<div class="code"><pre>
107	#define NOTE(str) \
108	message(__FILE__ "(" BOOST_PP_STRINGIZE(__LINE__) ") : " str) \
109	/**/
110
111	// ...
112
113	#pragma NOTE("TBD!")
114	</pre></div>
115	<div>
116	<b>Why:</b>  Macro expansion proceeds recursively in "layers."
117	Stringization prevents the preprocessor from performing macro expansion, therefore it is often necessary to delay stringization.
118	</div>
119
120	<h4>Example<u> - Use BOOST_PP_ENUM_PARAMS (and its variants) or BOOST_PP_REPEAT and BOOST_PP_COMMA_IF to avoid <i>O</i>(<i>n</i>) repetition on lists in general.</u></h4>
121	<div class="code"><pre>
122	struct make_type_list_end;
123
124	template<
125	BOOST_PP_ENUM_PARAMS_WITH_A_DEFAULT(
126	MAKE_TYPE_LIST_MAX_LENGTH,
127	class T,
128	make_type_list_end
129	)
130	>
131	struct make_type_list {
132	private:
133	enum { end = is_same<T0, make_type_list_end>::value };
134	public:
135	typedef typename type_if<
136	end, type_cons_empty,
137	type_cons<
138	T0,
139	typename type_inner_if<
140	end, type_identity<end>,
141	make_type_list<
142	BOOST_PP_ENUM_SHIFTED_PARAMS(
143	MAKE_TYPE_LIST_MAX_LENGTH,
144	T
145	)
146	>
147	>::type
148	>
149	>::type type;
150	};
151	</pre></div>
152	<div>
153	<b>How:</b>  BOOST_PP_REPEAT uses simulated recursion (pseudo code):
154	</div>
155	<div><pre>
156	#define BOOST_PP_REPEAT(n, m, p) BOOST_PP_REPEAT ## n(m, p)
157
158	#define BOOST_PP_REPEAT0(m, p)
159	#define BOOST_PP_REPEAT1(m, p) m(0, p)
160	#define BOOST_PP_REPEAT2(m, p) m(0, p) m(1, p)
161	#define BOOST_PP_REPEAT3(m, p) BOOST_PP_REPEAT2(m, p) m(2, p)
162	#define BOOST_PP_REPEAT4(m, p) BOOST_PP_REPEAT3(m, p) m(3, p)
163	// ...
164	</pre></div>
165	<div>
166	<i>Note:  This is no longer how BOOST_PP_REPEAT is implemented, so the code above is illustrative only! </i>
167	</div>
168	<div>
169	BOOST_PP_ENUM_PARAMS and its variations use BOOST_PP_REPEAT.
170	BOOST_PP_COMMA_IF(I) expands to a comma if I != 0.
171	BOOST_PP_INC(I) essentially expands to "I+1," and BOOST_PP_DEC(I) essentially expands to "I-1.".
172	</div>
173
174	<h4>Example<u> - Use a <i>conditional macro definition</i> to enable user configuration of code repetition based on need rather than some "reasonable" upper limit.</u></h4>
175	<div class="code"><pre>
176	#ifndef MAKE_TYPE_LIST_MAX_LENGTH
177	#define MAKE_TYPE_LIST_MAX_LENGTH 8
178	#endif
179	</pre></div>
180	<div>
181	Now the user can configure the <code>make_type_list</code> primitive without modifying library code.
182	</div>
183
184	<h4>Example<u> - Use BOOST_PP_REPEAT and a <i>token look-up function</i> to eliminate categorical repetition.</u></h4>
185	<div class="code"><pre>
186	// CAVEAT: My compiler is not standard on arithmetic types.
187	#define ARITHMETIC_TYPE(I) ARITHMETIC_TYPE ## I
188
189	#define ARITHMETIC_TYPE0 bool
190	#define ARITHMETIC_TYPE1 char
191	#define ARITHMETIC_TYPE2 signed char
192	#define ARITHMETIC_TYPE3 unsigned char
193	#define ARITHMETIC_TYPE4 short
194	#define ARITHMETIC_TYPE5 unsigned short
195	#define ARITHMETIC_TYPE6 int
196	#define ARITHMETIC_TYPE7 unsigned int
197	#define ARITHMETIC_TYPE8 long
198	#define ARITHMETIC_TYPE9 unsigned long
199	#define ARITHMETIC_TYPE10 float
200	#define ARITHMETIC_TYPE11 double
201	#define ARITHMETIC_TYPE12 long double
202
203	#define ARITHMETIC_TYPE_CNT 13
204
205	// ...
206
207	#define BOOST_PP_DEF(z, I, _) /* ... */ \
208	catch (ARITHMETIC_TYPE(I) t) { \
209	report_typeid(t); \
210	report_value(t); \
211	} \
212	/**/
213
214	BOOST_PP_REPEAT(ARITHMETIC_TYPE_CNT, BOOST_PP_DEF, _)
215
216	#undef BOOST_PP_DEF
217	</pre></div>
218	<div>
219	<b>Note:</b>  The repetition of the above example can be eliminated using template metaprogramming <a href="../bibliography.html#czarnecki">[Czarnecki]</a> as well.
220	However categorical repetition of operator tokens cannot be completely eliminated by using template metaprogramming.
221	</div>
222
223	<h4>Example<u> - Use BOOST_PP_REPEAT to avoid <i>O</i>(<i>n</i>*<i>n</i>) repetition.</u></h4>
224	<div class="code"><pre>
225	#ifndef MAX_VEC_ARG_CNT
226	#define MAX_VEC_ARG_CNT 8
227	#endif
228
229	// ...
230
231	#define ARG_FUN(z, i, _) BOOST_PP_COMMA_IF(i) T a ## i
232	#define ASSIGN_FUN(z, i, ) (*this)[i] = a ## i;
233
234	#define DEF_VEC_CTOR_FUN(z, i, _) /* ... */ \
235	vec(BOOST_PP_REPEAT(i, ARG_FUN, _)) { \
236	BOOST_PP_REPEAT(i, ASSIGN_FUN, _) \
237	} \
238	/**/
239
240	BOOST_PP_REPEAT(BOOST_PP_INC(MAX_VEC_ARG_CNT), DEF_VEC_CTOR_FUN, _)
241
242	#undef ARG_FUN
243	#undef ASSIGN_FUN
244	#undef DEF_VEC_CTOR_FUN
245
246	// ...
247	</pre></div>
248	<div>
249	<b>How:</b>  BOOST_PP_REPEAT is implemented is a special way to enable <i>automatic recursion</i>.
250	</div>
251
252	<h4>Example<u> - Use BOOST_PP_IF to implement special case for the first element.</u></h4>
253	<div class="code"><pre>
254	#define COMMA_IF(c) \
255	BOOST_PP_IF(c, BOOST_PP_COMMA, BOOST_PP_EMPTY)() \
256	/**/
257
258	BOOST_PP_IF(0, true, false) == false;
259	BOOST_PP_IF(1, true, false) == true;
260	</pre></div>
261	<div>
262	BOOST_PP_IF enables convenient generation of lists using BOOST_PP_REPEAT.
263	</div>
264	<div>
265	<b>Note:</b>  <i>THEN</i> and <i>ELSE</i> don't have to be macros.
266	However, if at least one of them is a function-like macro, and you want it be expanded conditionally,
267	you have to make the other parameter a function-like macro too.
268	This can often be done using BOOST_PP_IDENTITY.
269	Consider the following example (by Aleksey Gurtovoy):
270	</div>
271	<div><pre>
272	#define NUMBERED_EXPRESSION(i, x) /* ... */ \
273	BOOST_PP_IF( \
274	i, \
275	BOOST_PP_IDENTITY(x ## i) \
276	BOOST_PP_EMPTY \
277	)() \
278	/**/
279	</pre></div>
280	<div>
281	<b>Note:</b>  Like in the above implementation of COMMA_IF, the result of BOOST_PP_IF is often invoked and not the <i>THEN</i> and <i>ELSE</i> parameters.
282	If the parameters were invoked, the code would not expand correctly, because the BOOST_PP_EMPTY parameter would get expanded
283	to nothing before the <b>BOOST_PP_IF</b> would be properly expanded.
284	</div>
285	<div>
286	<b>How:</b>  BOOST_PP_IF is defined for the entire repeat range (psuedo code):
287	</div>
288	<div><pre>
289	#define BOOST_PP_IF(c, THEN, ELSE) BOOST_PP_IF ## c(THEN, ELSE)
290
291	#define BOOST_PP_IF0(THEN, ELSE) ELSE
292	#define BOOST_PP_IF1(THEN, ELSE) THEN
293	#define BOOST_PP_IF1(THEN, ELSE) THEN
294	// ...
295	</pre></div>
296
297	<h4>Example:<u>  Use arithmetic, logical, and comparison operations when necessary.</u></h4>
298	<div class="code"><pre>
299	#define SPECIAL_NUMBERED_LIST(n, i, elem, special) \
300	BOOST_PP_ASSERT_MSG( \
301	BOOST_PP_LESS(i, n), \
302	bad params for SPECIAL_NUMBERED_LIST! \
303	) \
304	BOOST_PP_ENUM_PARAMS(i, elem) \
305	BOOST_PP_COMMA_IF(i) special \
306	BOOST_PP_REPEAT( \
307	BOOST_PP_SUB(BOOST_PP_DEC(n), i), \
308	SPECIAL_NUMBERED_LIST_HELPER, \
309	(elem, i) \
310	) \
311	/**/
312
313	#define SPECIAL_NUMBERED_LIST_HELPER(z, i, elem_base) \
314	, \
315	BOOST_PP_CAT( \
316	BOOST_PP_TUPLE_ELEM(2, 0, elem_base), \
317	BOOST_PP_ADD( \
318	i, \
319	BOOST_PP_TUPLE_ELEM(2, 1, elem_base) \
320	) \
321	) \
322	/**/
323
324	SPECIAL_NUMBERED_LIST(3, 0, E, S)
325	SPECIAL_NUMBERED_LIST(3, 1, E, S)
326	SPECIAL_NUMBERED_LIST(3, 2, E, S)
327	SPECIAL_NUMBERED_LIST(3, 3, E, S)
328	</pre></div>
329
330	<hr size="1">
331	<div style="margin-left: 0px;">
332